Resistance to pyrethroid insecticides in soybean looper (Lepidoptera: Noctuidae) in Mississippi

Recommended rates of pyrethroid insecticides failed to give adequate control of soybean looper, Pseudoplusia includens (Walker), on soybean in so areas of the alluvial plain (Delta) region of Mississippi in 1987. Permethrin provided 53.0% control 5 d after application in an area of the Delta where permethrin control failures had been suspected in 1985 and 1986. This was lower than the 87.5% control reported in another area of the Delta where no permethrin control failures were suspected during these years. Numbers of soybean looper larvae were 69.4 and 73.4% lower in methomyl treatments and 58.5 and 56.2% lower in acephate treatments than in the untreated control in the Delta within and outside areas of suspected pyrethroid control failures, respectively. Compared with LD50's for a laboratory strain susceptible to pyrethroids, LD50's for permethrin applied to soybean loopers collected in the Delta were 10.4 and 21.1 times higher for two strains established from larvae collected in soybean fields treated with permethrin, 3.5 times higher for a strain from an untreated soybean held, and 7.3 times higher for a strain established from adults collected in an ultraviolet light trap. The levels of resistance of the Delta strains to cypermethrin and fenvalerate were similar to those for permethrin. LD50's of cypermethrin and permethrin for a strain established from the northeastern (Hill) region of Mississippi were not different from the LD50 of the susceptible strain. LD50's for methomyl applied to field strains were variable but were not different from the laboratory strain. The same was true for methyl parathion, except that a Delta strain collected from a soybean field treated with methyl parathion showed some evidence of resistance